The present invention relates to a multi-phase power system and in particular to the reliability of that system during a loss of phase voltage.
As widely known in the art, many protective devices, such as relays, circuit breakers, various types of monitoring, supervising and fault detection units, are used in power system. One basic task of these devices is to allow electrical power to be distributed in a reliable manner and to adequately protect either the transmission lines and the loads or equipments connected therewith, e.g. generators, motors, from hazardous conditions which could lead to malfunction, severe damage, failure, etc.
Often, the protection of power system elements is based on accurate measurement of three-phase voltages in order to provide reliable fault detection and breaker operations and thus minimize power system disruptions. In these cases, incorrectly measuring one or more of the three-phase voltages by a protective unit may result in technical shortcomings, e.g. erroneous trips (breaker operations) and/or clearing more of the power system than desired.
For example a common failure that causes incorrect voltage measurement is when there is a downed line, or due to an equipment failure in an end-user's facility, or when one or more fuses protecting a three-phase voltage transformer (VT) secondary circuit blow. In the latter case, protective relays connected to that secondary circuit would measure zero voltage if a secondary phase is isolated (only phase-to-ground connections) or there exist a non-zero coupled value if there are phase-to-phase connections in the secondary circuit. Conditions, other than blown fuses, may also occur where one or more secondary phase voltages are unintentionally removed from the protective relay.
In the electrical power system industry operating in this abnormal state may for example be referred to as “single-phasing”, or as fuse failure, or as “loss-of-potential (LOP)”, or more commonly as “loss-of-voltage (LOV)” and this last definition (“LOV”) will be used hereinafter to refer to the loss of voltage in a single phase.
It is clear that when an LOV state occurs, adequate control of voltage dependent measuring units need to be carried out so as to reduce potential detrimental effects on the whole power system and/or on the various pieces of equipment.
For example, with protection measuring units, such as distance and impedance units, e.g. distance relays, functioning is based on calculating the impedance seen by the units themselves; since the impedance measured is directly dependent on the voltage(s). Thus an LOV condition will adversely affect obtaining correct measurements and therefore missing one or more phase voltages will lead to misoperations, such as untimely tripping.
The same considerations apply also in the case of the so-called directional units, i.e. protection measuring devices that determine the direction of the current flow in an AC circuit and are used to supervise for example an overcurrent relay in order to let it trip only in the desired direction. These directional units can perform their supervision task by comparing the angular relationship between the current in the protected circuit and an independent voltage source. Since the current can vary significantly for various types of faults, in order to determine directionality the independent voltage may be used as a reference or polarizing quantity. This reference voltage may be not correct during an LOV state, and hence the directional units will not be able to provide the required supervision and may contribute to a misoperation.
A further example of the negative impact that the occurrence of an LOV state may have on power line protection is the operation of a recloser. A recloser opens and closes multiple times when a fault condition exists in an attempt to clear the fault. Should the fault condition continue to exist, the recloser opens and remains open until reset manually. The recloser enters a “lock out” state when this occurs. Automatic reclosing requires synchronization or comparison of voltage on each side of an open line circuit breaker. These comparisons are often accomplished by comparing the voltage from one phase VT on each side of the open breaker. If an LOV condition exists on either VT, reclosing may operate incorrectly and prevent automatic system restoration, cause a line outage, or possibly cause circuit breaker damage. Thus, the location of the LOV may impact the protection functions provided by the recloser.
In the past, in order to improve reliability under an LOV state, various solutions have been devised varying depending on several aspects of the specific applications, such as type, number and technology of protecting systems, available options provided by the units, architecture of the power system, etc. These known solutions, even though providing some improvements may not be entirely satisfactory as they may, when an LOV condition occurs, either give rise to false trip or not allow for local fault clearing, or require the use of costly redundant protection system.
Thus it is desirable to provide a solution which improves operational reliability during an LOV state.